They all die to Shock, Lightning Bolt, and Burst Lightning. All of which see more gameplay then Tremor. So let's not bother saying which cards can kill these guys. Gemhide Sliver does tend to be better though. Chanses are that if you are using a Gemhide Sliver then you have many slivers which would gain the ability.

Not quite as good as Birds of Paradise, but still really good and can actually block something and survive. This would work well if you already had 4 BoPs, so you could have 8 any color mana producing cheap creatures. Gemhide Sliver would also work, but it only has 1 toughness.

I like the tree. Sure, it's slightly worse than most other "generate mana of any color" cards. But... uhm... well, it's not much worse. :| And it fits into your plant tribal deck. I guess I like it because it's from a block I like a lot.